1. Field of the Invention
This invention relates to measuring the transfer function of optical devices. More specifically, the invention is a system and method for measuring the transfer function associated with single-port guided wave device or the transfer function matrix of a multi-port guided wave device, e.g., Bragg gratings, couplers, etc.
2. Description of the Related Art
A variety of guided wave devices (e.g., Bragg gratings, directional couplers, isolators, amplitude modulators, amplifiers, wave division multiplexers, etc.) are used in the world""s telecommunication network. In order to understand and predict how these devices will affect an incoming (light) signal, it is necessary to characterize the impulse response (i.e., know the transfer function or transfer function matrix) of these devices. Current systems/methodologies for measuring such transfer functions are very expensive, slow, and/or subject to unsatisfactory levels of error.
One commercially available impulse response characterization system is shown in FIG. 1 where light from a tunable laser 10 undergoes amplitude modulation at 12 as controlled by a fixed frequency oscillator 14. The modulated light is directed (by an optical coupler 16) down an optical fiber 18 to a device under test (DUT) 20 such as a Bragg grating. The light will at least partially reflect off DUT 20 and pass back through optical coupler 16 where it is directed to a detector 22. The (modulated) reflection signal at detector 22 is recovered with some phase shift as measured by a vector volt meter 24. Due to the nature of DUT 20, the delay experienced by the reflected signal is a function of the center wavelength of tunable laser 10. The delay as a function of this center wavelength (i.e., dispersion measurement) is used to characterize DUT 20. However, this system is expensive and can take twenty minutes to test a single device.
Accordingly, it is an object of the present invention to provide a system for measuring transfer functions of guided wave devices.
Another object of the present invention is to provide a simple and inexpensive system for measuring transfer functions of a variety of guided wave devices.
Still another object of the present invention is to reduce the time required to measure transfer functions associated with guided wave devices.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a method and system are provided for measuring the Nxc3x97N scalar transfer function elements for an N-port guided wave device. The device has a maximum effective path length LD and an effective index n. Optical energy of a selected wavelength is generated at a source and directed along N reference optical paths having N reference path lengths LRi, i=1 to N. Each reference optical path terminates in one of N detectors such that N reference signals are produced at the N detectors. The reference signals are indicative of amplitude, phase and frequency of the optical energy carried along the N reference optical paths. The optical energy from the source is also directed to the N-ports of the guided wave device and then on to each of the N detectors such that N measurement optical paths are defined between the source and each of the N detectors. A portion of the optical energy is modified in terms of at least one of the amplitude and phase to produce N modified signals at each of the N detectors. At each of the N detectors, each of the N modified signals is combined with a corresponding one of the N reference signals to produce N combined signals at each of the corresponding N detectors. A total of N2 measurement signals are generated by the N detectors. Each of the N2 measurement signals is sampled at a wave number increment xcex94k so that N2 sampled signals are produced. In the present invention, it is required to define N measurement path lengths LMj, j=1 to N, of the N measurement optical paths associated with each of the N detectors such that
|LRixe2x88x92LMj| greater than N*LD|j=1,N |i=1,Nxe2x80x2
                                                                        "LeftBracketingBar"                                                      L                    Ri                                    -                                      L                    Mj                                                  "RightBracketingBar"                            +                              L                D                                       less than                           π                              2                ⁢                n                ⁢                                  xe2x80x83                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                k                                              "RightBracketingBar"                                      j            =            1                    ,          N                    "RightBracketingBar"                      i        =        1            ,      N        ,
and such that the N combined signals at each of the N detectors are spatially separated from one another in the time domain. The Nxc3x97N transfer function elements are generated using the N2 sampled signals.